As a processing speed of an electronic device has been increased, densification of signal wiring connecting the electronic device with the others with the increase in signal speed have become technical challenges. In conventional electric signal transmission technology, as the speed of signal increases, a dielectric loss and electromagnetic radiation in a periphery of the wiring are increasing. For this reason, in order to realize transmission speed equal to or more than several gigabits per second, a technical design is required for selection of a dielectric material, wiring density of transmission channels and the like, and the wiring becomes subject to physical restrictions.
In order to avoid such restrictions and to realize signal transmission at higher speed and with higher density, optical wiring by means of an optical waveguide and an optical fiber (hereinafter, collectively referred to as an “optical waveguide”) has been being studied. Here, for the purpose of supply of a power source for driving the electronic device and of low-speed signal transmission, it becomes necessary to make electric wiring coexist simultaneously with the optical wiring because the optical wiring has small economic advantage.
Japanese Patent Laid-Open No. 2000-227524 (hereinafter referred to as “Patent Document 1”) and Japanese Patent Laid-Open No. 2000-235127 (hereinafter referred to as “Patent Document 2”) disclose optical waveguide structures in each of which the electric wiring and the optical waveguide structure are mixed. In Patent Document 1, the optical waveguide is formed on a board, and on the optical waveguide, a light emitting element and a light receiving element are provided. Then, light emitted from the light emitting element is reflected on one end of the optical waveguide substantially at a right angle, propagated through the optical waveguide, reflected on the other end substantially at a right angle, and made incident onto the light receiving element. The optical waveguide structure of Patent Document 1 is constructed in a manner as described above. In Patent Document 2, while the light emitting element and the light receiving element are provided on an upper surface of the board, the optical waveguide is provided on a lower surface of the board, the light emitted from the light emitting element is guided to the lower surface of the board, and propagated through the optical waveguide. Then, the light is reflected on an end portion of the optical waveguide, and made incident onto the light receiving element located on the upper surface of the board. The optical waveguide structure of Patent Document 2 is constructed in a manner as described above.
There is a possibility that the optical waveguide, for use in the board in which the electric wiring and the optical waveguide structure are mixed, may be exposed to high temperature in a manufacturing process of an electric wiring board and an mounting process of components, and may be subjected to a mechanical impact and the like in use of the completed board. Therefore, it is preferable that the optical waveguide be mounted in an inner layer of the board to the extent possible. Furthermore, in order to avoid an influence of warp of the board, a structure is preferable, in which the optical waveguide is disposed at the center of the board, and boards on both sides of the optical waveguide, which sandwich the optical waveguide, are made of materials equal in thermal expansion coefficient and thickness so as to be arranged symmetrically to each other in a thickness direction.
More specifically, the board is heated in steps shown below in the manufacturing process of the laminated electric wiring board.
In a laminating step of the boards, the boards are stacked with adhesive resin being sandwiched therebetween, and the resin is cured for a few hours at temperature of a hundred and several ten degrees centigrade (180 to 190 degrees centigrade) with pressure. With regard to heat resistance of resin for use as a material of the optical waveguide, in general, phase transition temperature or glass transition temperature using a change of mechanical impedance as an index is conceived as a measure. However, when the resin is heated on exposure to air, a degradation in optical characteristics, such as yellowing, may sometimes be brought by a thermochemical reaction with the air though mechanical strength of the resin is maintained. Hence, in order to prevent such a degradation, it is effective to shield the optical waveguide from the air by disposing the waveguide not on a surface layer of the board but in the inner layer to the extent possible.
Moreover, in the step of mounting (assembling) components on the board, soldering process is performed in almost all the cases. In recent years, solder of lead-free type has used in consideration of the environment, and temperature of approximately 260° C. in the soldering process has become higher than the conventional temperature. Optically transparent acrylic resin and the like, which have heat resistance even to such a relatively high temperature, have been announced, and in the future, problems regarding the heat resistance will be reduced pretty much by using such resins. However, it is thought to be difficult to avoid the degradation of the optical characteristics due to the thermal reaction with the air, and also in order to solve this process problem, it is effective to dispose the optical waveguide in the inner layer of the board. Then, a time for the soldering process is relatively short, and accordingly, if the optical waveguide is disposed in the inner layer protected by resin layers in which heat conduction is relatively small, temperature increase in the inside can be restricted.
As described above, instead of adopting the structure in which the optical waveguide is exposed on the surface of the board as in Patent Documents 1 and 2, the disposition of the optical waveguide in the innermost layer of the board has a great practical advantage in terms of avoiding the problems in the manufacturing process and the assembly. However, in this structure, the optical waveguide in the laminated board and a light receiving/emitting element mounted on the surface of the board will be arranged to be spaced at a distance corresponding to the thickness of the board. For this reason, an optical component connection technology will be required. In this technology, an array of high density optical signal transmission paths is disposed in the inner layer of the laminated wiring board. Then, the light receiving/emitting element, the light receiving/emitting board, and the like, which are mounted on the board, and the array of the high density optical signal transmission paths, are optically connected to each other at a distance ranging from several ten microns to several millimeters.
Specifically, when the optical waveguide is disposed in the inner layer portion of the board, a structure for changing the direction of the light and a structure for guiding the light to the surface of the board will be required between the optical waveguide and the light receiving/emitting element which is mounted on the surface of the board. Here, when the light is propagated in the air between the optical waveguide and the light receiving/emitting element, the most part of the light does not reach the light receiving element but is dispersed in a periphery thereof, and coupling efficiency is significantly lowered. Furthermore, when the optical wiring is mounted with high density, the light is received by adjacent light receiving elements, causing a large interchannel crosstalk. Hence, in such a structure, it is difficult to realize optical connection in which, with regard to the density of the optical wiring, a wiring pitch is shorter than 250 microns realized by the current fiber ribbon, and the connection distance between the optical waveguide and the light receiving/emitting element is set in a range from several ten microns to several millimeters.
In order to avoid this problem, in Patent Document 2, a lens is used between the optical waveguide and the light receiving/emitting element. However, a divergence angle (numerical aperture: NA) of the light of the multimode fiber or optical waveguide or of the light of the light emitting element is generally 0.2 or more, and when the connection distance l is set at 1 mm, a relationship is unwillingly established as:l·sin θ=l·NA=0.2 mm≈wiring pitch
Therefore, signal separation between the adjacent lines of wiring becomes difficult.